Installation for Welding Frameworks of Nuclear Fuel Assemblies, Programming Method, Corresponding Methods for Framework Welding and Assembling

ABSTRACT

The installation provided includes at least one structure for receiving and holding guide tubes and structural elements; a carriage movable parallel to the guide tubes; at least one welding tool; and displacement means for moving the welding tool, the displacement means connecting the pincer to the carriage and presenting at least six degrees of freedom.

The present invention relates to an installation for welding structuralelements onto guide tubes of a nuclear fuel assembly skeleton, theinstallation comprising:

-   -   at least one structure for receiving and holding guide tubes and        structural elements;    -   a carriage movable parallel to the guide tubes;    -   at least one welding tool; and    -   displacement means for moving the welding tool, the displacement        means connecting the pincer to the carriage and presenting at        least five degrees of freedom.

The invention applies in particular to welding grids for holding nuclearfuel rods.

FR-2 670 947 discloses an installation of the above-specified type thatenables grids to be welded that are provided with welding tonguesprojecting from the top faces of the grids. The displacement means ofthe welding installation comprise a support box that is movabletransversely relative to the guide tubes, a vertically-extensible arm,and a steerable wrist that carries a welding pincer. The steerable wristis capable of moving in rotation about three distinct axes, such thatthe welding pincer presents a total of six degrees of freedom. Theabove-described installation makes it possible to achieve high rates ofwelding throughput, but it is still desirable to increase those rates.

U.S. Pat. No. 4,587,394 also discloses a welding installation, whichinstallation has four welding tools moving simultaneously so that eachcontributes to welding grids to the guide tubes of a given skeleton.Each tool is connected to a carriage that is movable longitudinallyrelative to the guide tubes by displacement means that present threedegrees of freedom. Although the four welding tools move simultaneously,the rates of throughput achieved by such a welding installation arelikewise found to be too slow.

SUMMARY OF THE INVENTION

An object of the invention is to solve that problem by providing aninstallation of the above-specified type that makes it possible toachieve faster rates of welding.

To this end, the invention provides an installation for weldingstructural elements onto guide tubes of a nuclear fuel assemblyskeleton, the installation comprising:

-   -   at least one structure for receiving and holding guide tubes and        structural elements;    -   a carriage movable parallel to the guide tubes;    -   at least one welding tool; and    -   displacement means for moving the welding tool, the displacement        means connecting the welding tool to the carriage and presenting        at least five degrees of freedom;    -   the installation being characterized in that the displacement        means present at least six degrees of freedom.

The invention also provides a programming method.

The invention also provides a method of welding a nuclear fuel assemblyskeleton.

The invention also provides a method of making a nuclear fuel assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood on reading the followingdescription given purely by way of example and made with reference tothe accompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view showing a welding installationof the invention, the structure for receiving the skeletons not beingshown;

FIG. 2 is a diagrammatic face view of the FIG. 1 installation, seenlooking along the direction of arrow II in FIG. 1;

FIG. 3 is a fragmentary diagrammatic view in perspective showing aportion of the skeleton of a nuclear fuel assembly that can be welded bythe installation of FIG. 1;

FIGS. 4 and 5 are fragmentary diagrammatic views in perspective showingthe means for moving the welding tool in the installation of FIGS. 1 and2; and

FIG. 6 is a fragmentary diagrammatic view in perspective showing awelding installation in another embodiment.

DETAILED DESCRIPTION

FIGS. 1 and 2 are diagrams showing an installation 1 for weldingstructural elements onto guide tubes of nuclear fuel assembly skeletons.

More precisely, the structural elements considered below are grids forholding nuclear fuel rods, even though other structural elements, suchas sleeves, could also be welded by the installation 1.

With reference to FIG. 3, it is recalled that a fuel assembly 2 mainlycomprises nuclear fuel rods 3 and a structure or skeleton 4 forsupporting the rods 3.

Conventionally, the support skeleton comprises:

-   -   a bottom nozzle 5 and a top nozzle 6 disposed at the        longitudinal ends of the assembly 1;    -   guide tubes 7 for receiving the rods of a cluster (not shown)        for controlling and stopping the nuclear reactor; and    -   grids 8 for holding the rods 3.

The nozzles 5 and 6 are secured to the longitudinal ends of the guidetubes 7.

The rods 3 extend vertically between the nozzles 5 and 6. The rods 3 aredisposed at the nodes of a substantially regular square-based array inwhich they are held by the grids 8. Some of the nodes of the array areoccupied by the guide tubes 7, and possibly by a central tube forinstrumentation purposes.

Conventionally, the grids 8 comprise sets of crossed plates that definebetween them cells that are centered on the nodes of the regular array.Most of the cells are for receiving a respective fuel rod 3. Each of theother cells receive a guide tube 7, and the central cell optionallyreceives an instrumentation tube 14.

Usually, the grids 8 are provided with welding tabs that project upwardsand that enable the grids 8 to be welded to the guide tubes 7. There arealso exist grids that do not have such tabs, as shown in U.S. Pat. No.4,849,161. The double plates of the grid in that document then presenttop setbacks defining zones for welding the grid to the guide tubes 7.

Like the installation of document FR-2 670 947, the installation 1 ofFIGS. 1 and 2 comprises an assembly bench 10 suitable for being coveredlaterally by protective panels. The bench 10 is constituted by a metalgantry comprising uprights and two horizontal longitudinal rails 11 onwhich there travels a carriage 12 carrying a welding robot 13.

The bench 10 defines two parallel work zones (FIG. 2), each at least aslong as a skeleton 4 for welding, and disposed side by side. In each ofthe zones there is placed a structure 14 enabling a skeleton 4 to bepreassembled thereon. Each structure 14 extends parallel to the rails 11and may comprise a machine-welded table 16 with a surface slab 18.

As shown in FIG. 2, the bench 10 also has rails 22 disposed transverselyto the rails 11 and enabling a cage 24 to move between the position inwhich it is shown in FIG. 2, where it is facing one of the structures14, and another position where it is facing the other structure 14. Thecage 24 is provided with longitudinal rails on which there travels acarriage 25 for positioning expandable mandrels that are to be insertedin the guide tubes 7 at the welding locations in order to prevent thetubes becoming deformed. These mandrels may have the structure asalready described in document FR-A-2 533 353 and they may be guided byframes carried by the cage (not shown).

The surface slab 18, which extends over the entire length of thestructure 14, is designed to receive, at adjustable locations, frames 20for positioning and holding the grids 8 of the skeleton that is to bewelded. These frames 20, are provided with means enabling them to beopened in order to insert a grid 8 and enabling them to be reclosed, andthey may be of conventional structure. In the example shown, the frames20 for receiving the grids 8 of a skeleton 4 have their diagonalsextending horizontally and vertically.

The carriage 12 occupies the entire width of the assembly bench andcarries at least one transverse rail 34.

The welding robot 13 has a resistance welding pincer 36 and displacementmeans 38 for moving the welding pincer 36 that connect the pincer 36 tothe carriage 12.

The displacement means 38 comprise:

-   -   a support box 40 that is movable transversely on the rail(s) 34;    -   an articulated arm 42 extending the support box 40 downwards;        and    -   a steerable wrist 44 extending the arm 42 and carrying the        welding pincer 36.

The support box 40 may contain the welding transformer for feedingelectricity to the welding pincer 36.

By means of the carriage 12, it is possible to move the welding pincer36 relative to the bench 10 in translation in the X direction asrepresented by double-headed arrow A in FIG. 1, i.e. parallel to theguide tubes 7 of a skeleton placed on a structure 14. Because of thesupport box 40, it is also possible to move the pincer 36 in translationalong the Y direction, i.e. transversely relative to the guide tubes 7,as represented by double-headed arrow B in FIG. 1.

As shown in FIGS. 4 and 5, the arm 42 comprises three successivesegments 46, 48, and 50. The first segment 46 extends the support box 40downwards. The second segment 48 is articulated to the first segment 46via a double rotary connection enabling both rotary movement about avertical axis, as represented by double-headed arrow C in FIG. 4, androtary movement about a horizontal axis, as represented by double-headedarrow D in FIGS. 4 and 5.

The third segment 50 is articulated to the second segment 48 of the arm42 by a double rotary connection enabling both relative rotary movementabout a horizontal axis as represented by double-headed arrow E in FIG.5, and relative rotary movement about the longitudinal axis of the thirdsegment 50, as represented by double-headed arrow F in FIGS. 4 and 5.

The wrist 44 carries the case 52 of the welding pincer 36. The case 52is provided with the usual electrical and pneumatic power supply means,constituted by cables. The wrist 44 is connected to the third segment 50of the arm 42 via a double rotary connection enabling both relativerotary movement about an axis that is orthogonal to the longitudinalaxis of the third segment 50, as represented by double-headed arrow G inFIGS. 4 and 5, and rotary movement about the longitudinal axis of thepincer 36, as represented by double-headed arrow H in FIGS. 4 and 5.

Thus, the welding pincer 36 presents eight degrees of freedomrepresented by arrows A to H, with the displacement means 38 providingseven of them.

As shown in FIG. 2, the installation 1 further includes programmablemeans 54 for controlling the displacement and the operation of thepincer 36. By way of example, these control means 54 comprise a computerhaving one or more processors, data storage means 56, input and outputmeans, and display means (not shown).

The input/output means comprise, for example, a user interface, e.g. inthe form of a controller 58 enabling the welding pincer 36 to be movedand operated under manual control.

In order to program the control means 54, which means are intendedsubsequently to control the operation of the welding pincer 36automatically, it is possible to proceed as follows, by way of example.

An operator uses the controller 58 to control the displacement of thepincer 36 manually until it reaches a first location for welding a firstgrid 8 to one of the guide tubes 7 of a skeleton 4 that has beenpreassembled on one of the structures 14, and then the operator causes awelding operation to be performed at this location. The electrodes ofthe pincer 36 then press the corresponding zone 9 of the grid 8 againstthe guide tubes 7 in question, thus performing welding by resistance.The coordinates of this first welding location, e.g. delivered byposition sensors 60 present in the displacement means 38 are thenrecorded in the storage means 56. Conventional sensors are shown verydiagrammatically in FIG. 1.

Thereafter, the operator continues with the welding sequence for all ofthe first grid 8 in question, the storage means 56 progressivelyrecording the coordinates of the various locations where welding isperformed, and also the positions of the pincer 36 during those weldingoperations. This sequence for welding a grid 8 is merely a portion ofthe complete sequence for welding the skeleton 4, and it is thereforereferred to below as a “sub-sequence”.

Thereafter, the operator brings the pincer 36 to the first location forwelding a second grid 8 to the guide tubes 7 of the skeleton 4. Theoperator can then trigger execution of the welding sub-sequence aspreviously carried out manually on the first grid 8 and recorded.

By proceeding in this way for each of the grids 8, the storage means 56record the data needed for performing all of the welding operations thatneed to be performed on all of the grids present along the guide tubes7.

This complete sequence for welding the grids 8 of a skeleton 4 is thenstored and can be executed automatically when a skeleton 4 of the sametype is to be welded by the installation, with the operator then needingonly to position the pincer 36 on the first location for welding thefirst grid 8 and then triggering execution of the entire sequence.

It is also possible to store, e.g. in the same manner as describedabove, complete sequences in the storage means 56 for welding skeletonsof other types.

This programming method, based on recording a training sequence carriedout under manual control, is very user-friendly and can be implementedvery quickly. In particular, it is less time-consuming than prior artmethods that require all of the coordinates of all of the weldingpositions to be defined one by one. Nevertheless, programming in waysother than that described above can also be used with the installation1. Conversely, this programming technique by training can be used withan installation 1 in which the number of degrees of freedom for thepincer 36 is arbitrary, e.g. six as in FR-2 670 977.

It should also be observed that the complete programmed weldingsequences may include sub-sequences that are different for some of thegrids 8, and also a sub-sequence for welding guide tubes to one of thenozzles 5 or 6 or to connection elements belonging to one of the nozzles5 or 6.

Because of the numerous degrees of freedom presented by the pincer 36,it has been found that welding sequences can be performed much morequickly than in the prior art. Surprisingly, it is better to increasethe number of degrees of freedom than to increase the number of weldingtools.

This is because shorter paths can be followed between the variouswelding locations.

In addition, because of the extra degrees of freedom in rotation, and inparticular because of the possibility of rotating about a horizontalaxis as represented by double-headed arrow D, the pincer 36 can move atspeeds that are very high, and in particular greater than 13 meters persecond (m/s).

In addition, because of the large number of degrees of freedom, thepincer 36 can reach zones that are difficult of access, and inparticular zones for welding grids of the kind disclosed in U.S. Pat.No. 4,849,161. The installation 1 can therefore be used for weldinggrids 8 of all types, and in particular grids that do not present anywelding tabs.

Furthermore, the pincer is capable of applying high welding forces, e.g.of the order of 60 decanewtons (daN), even in locations that aredifficult of access.

As in FR-2 670 947, the presence of two structures 14 makes it possibleto use one for preassembling a skeleton while the other one is beingused with the pincer 36 to weld together the elements of a skeleton 4that has already been preassembled. By operating in parallel in thisway, the welding robot 13 is used on an almost continuous basis.

Nevertheless, in certain variants, the installation 1 could have onlyone structure 14.

In general, the displacement means 48 can present fewer degrees offreedom, for example only seven.

Similarly, the degrees of freedom are not necessarily those describedabove. It is possible to envisage using combinations of movements inrotation and in translation other than those described above.

FIG. 6 shows another embodiment of a welding installation 1 that differsfrom that described above mainly as follows.

The gantry 10 is mounted to slide on the structure 14 that receives askeleton for welding. The support box 40 is secured to the top web 62 ofthe gantry 10 and therefore does not present any degree of freedomrelative thereto. The double-headed arrow B of the above-describedembodiment is therefore not shown in FIG. 6.

The connection between the segments 48 and 50 of the arm 42 is a simplerotary connection, represented by double-headed arrow E. The connectionbetween the segment 50 and the wrist 44 is a triple rotary connection,represented by double-headed arrows F to H.

The displacement means 38 for moving the tool 36 thus present sixdegrees of freedom represented by the arrows C to H.

Mounting the gantry 10 on the structure 14 thus enables one degree offreedom to be eliminated, specifically the degree of freedom representedby the arrow B in the above figures.

In addition, mounting the gantry 10 on the structure 14 improves thepositioning of the tool 36 relative to a skeleton for welding.

It should be observed that this characteristic can be used independentlyof the number of degrees of freedom of the displacement means 38.

Thus, a welding installation 1 may have a gantry 10 slidably mounted ona structure 14, without its displacement means 38 presenting at leastsix degrees of freedom.

In general, the welding tool 36 can implement welding of some type otherthan resistance welding.

1 to
 14. (canceled)
 15. An installation for welding structural elementsonto guide tubes of a nuclear fuel assembly skeleton, the installationcomprising: at least one structure for receiving and holding guide-tubes and structural elements; a carriage movable parallel to the guidetubes; at least one welding tool; and displacement means for moving thewelding tool, the displacement means connecting the welding tool to thecarriage and presenting at least six degrees of freedom.
 16. Aninstallation according to claim 15, wherein the displacement meanspresent at least seven degrees of freedom.
 17. An installation accordingto claim 15, wherein the displacement means comprise an arm having atleast two segments articulated one to another, and wherein thedisplacement means further comprise a wrist that carries the weldingtool, the wrist extending the arm and being articulated to the arm via aconnection presenting at least two degrees of freedom.
 18. Aninstallation according to claim 17, wherein the wrist is articulated tothe arm by a connection enabling movement in rotation about at least twodistinct axes.
 19. An installation according to claim 17, wherein thearm comprises at least three segments articulated one to another.
 20. Aninstallation according to claim 17, wherein the at least two segments ofthe arm are articulated one to another via respective connectionsallowing movement in rotation about two distinct axes.
 21. Aninstallation according to claim 15, wherein the displacement meanscomprise a support box movable transversely to the guide tubes.
 22. Aninstallation according to claim 15, wherein the carriage is mounted tomove on the structure.
 23. An installation according to claim 15,wherein the welding tool is a resistance-welding pincer.
 24. Aninstallation according to claim 15, further comprising a programmablecontroller controlling the displacement means for moving the weldingtool, and a manual controller controlling the displacement means formoving the welding tool.
 25. An installation according to claim 24,wherein the displacement means include position sensors.
 26. A method ofprogramming an installation according to claim 24, the method comprisingthe steps of: manually controlling the displacement means so that thewelding tool executes a welding sequence; and recording the weldingsequence in the programmable control means.
 27. A method of welding anuclear fuel assembly skeleton, the skeleton comprising guide tubes andstructural elements welded to the guide tubes, the method comprising thesteps of welding structural elements to the guide tubes using aninstallation according to claim
 15. 28. A method of making a nuclearfuel assembly, the assembly comprising nuclear fuel rods and a skeletonfor supporting the rods, the skeleton comprising guide tubes andstructural elements welded to the guide tubes, the method using askeleton welding method according to claim
 26. 29. An installation forwelding structural elements onto guide tubes of a nuclear fuel assemblyskeleton, the installation comprising: at least one structure forreceiving and holding guide tubes and structural elements; a carriagemovable parallel to the guide tubes; at least one welding tool; and aconnector connecting the welding tool to the carriage and capable ofmoving the welding tool with at least six degrees of freedom.